Method for making boron nitride fibres from aminoborazines

ABSTRACT

The invention concerns a method for making boron nitride fibres by drawing a polymer precursor and treating with ceramics the polymer fibres obtained by drawing. The invention is characterized in that the precursor polymer is obtained by thermal polymerization of a borazine of formula (I) wherein: R 1 , R 3 , R 4  and R 5 , identical or different, represent an alkyl, cycloalkyl or aryl group; and R 2  represents a hydrogen atom or an alkyl, cycloalkyl or aryl group.

DESCRIPTION

[0001] 1. Technical Domain

[0002] The purpose of this invention is a process for manufacturingboron nitride fibres, and particularly continuous boron nitride fibreswith good mechanical properties that can be used to make ceramiccomposite materials such as BN/BN composites, thermostructural parts andantenna radomes.

[0003] More precisely, it concerns obtaining boron nitride fibres from apolymer precursor that is shaped by spinning to form polymer fibres thatare then ceramised to transform them into boron nitride fibres.

[0004] 2. State of Prior Art

[0005] There are many processes for making boron nitride, as describedby R. T. PAINE et al in chem. Rev., 90. 1990, pages 73-91 [1]. Inparticular, the methods described in this document include processesusing precursor polymers formed from organic boron compounds such asborazines.

[0006] One way of obtaining this type of precursor polymers wasdescribed by C. K. Narula et al in Chem.. Mater, 2, 1990, pages 384-389[2]. It consists of making trichloroborazine or 2-(dimethylamino)-4,6-dichloroborazine react with hexamethyldisilazane in solution indichloromethane at ambient temperature.

[0007] If 2- (dimethylamino) -4,6-dichloroborazine is used,polymerisation at two points is encouraged due to the presence of theNMe₂ group.

[0008] Another method of obtaining precursor polymers described inEP-A-0 342 673 [3] consists of making a B-tris (inferior amino alkyl)borazine react with an alkylamine such as laurylamine, either thermallyin mass or in solution.

[0009] Other precursor polymers can also be obtained by thermalpolycondensation of trifunctional aminoborazines with formula[—B(NR¹R²)—NR³—]₃ in which R¹, R² and R³ are identical or different andrepresent hydrogen, an alkyl radical or an aryl radical as described inFR-A-2 695 645 [4].

[0010] The polymers described above are suitable for obtaining powder orother forms of boron nitride, but it is more difficult to prepare morecomplex forms, and particularly fibres from this type of polymers.

[0011] Frequently, the precursor polymer necessary for shaping thefibres is drawn badly due to its statistical reticulated structure whichcauses only a slight elongation, making control of the fibre sectionvery random. Later on in the process, this causes breakages of fibres orweak points, which results in very weak final mechanical properties.

[0012] As indicated by T. Wideman et al in Chem. Mater., 10, 1998, pp.412-421 [5], research has been continued to find other precursorpolymers that are more suitable for obtaining boron nitride fibres. Thisdocument describes that a spinnable precursor polymer in the moltenstate may be obtained by modifying polyborazylene by reaction with adialkylamine or with hexamethyldisilazane.

PRESENTATION OF THE INVENTION

[0013] The purpose of this invention is a process for manufacturingboron nitride fibres using other precursors to obtain fibres withsatisfactory mechanical properties.

[0014] According to the invention, this result is achieved using aborazine in which the three boron atoms are substituted by amino groups,at least one of which is different, as the precursor monomer.

[0015] According to the invention, the process for manufacturing boronnitride fibres by spinning of a precursor polymer and ceramisation ofthe polymer fibres obtained by spinning, is characterised in that theprecursor polymer is obtained by thermal polymerisation of a borazine offormula (I):

[0016] in which R¹, R³, R⁴ and R⁵ that may be identical or different,represent an alkyl, cycloalkyl or aryl group, and

[0017] R² represents a hydrogen atom or an alkyl, cycloalkyl or arylgroup.

[0018] In this process, the choice of a borazine with formula (I) toform the precursor polymer leads to an approximately linear polymer. Thefact that the borazine used is an asymmetric borazine concerning aminogroups present on its boron atoms, encourages links between monomerpatterns along two lines so that a reticulated polymer is not obtained,inducing a proportion of direct intercyclic links in the polymer.

[0019] In the borazine used in the invention, the R¹ to R⁵ groups mayrepresent alkyl, cycloalkyl or aryl groups. Alkyl and cycloalkyl groupsmay have 1 to 30 carbon atoms, and preferably from 1 to 10 and evenbetter 1 to 4 atoms of carbon. For ceramisation, it is preferable tolimit the number of carbon atoms in substitutes to obtain a betterconversion rate to boron nitride.

[0020] Aryl groups that could be used in the invention may be groupscomprising one or several phenyl radicals, and phenyl and benzyl groupsare used in preference.

[0021] According to one preferred embodiment of the invention, R² informula (I) represents a hydrogen atom. The result is then adysfunctional precursor comprising two NHR amino groups where R is analkyl, cycloalkyl or aryl group, and a tertiary amino group. Thisarrangement is favourable for obtaining a polymer with better spinningperformances.

[0022] Also preferably, the remaining R¹, R³, R⁴, R⁵ groups are methylgroups since they facilitate good ceramic efficiency.

[0023] Also according to a first embodiment of the invention, borazinecomplies with formula (I) in which R² represents a hydrogen atom and R¹,R³, R⁴, and R⁵ represent the methyl group. Therefore, this is[2,4-bis(monomethylamino)-6-dimethylamino]borazine.

[0024] According to a second embodiment of the invention, borazinecomplies with formula (I) where R¹ to R⁵ represent the methyl groupcorresponding to [2,4-bis(dimethylamino)-6-monomethylamino]borazine.

[0025] These borazines may be synthesised by the process described by B.Toury et al in Main Group Met. Chem. 22, 1999, pp. 231-234 [6]. In thisdocument, it was shown that polymerisation of borazines of the same typeat moderate temperatures (140 to 145° C.) leads to polymers with directB-N links between two borazine radicals. On the other hand, linearity ofthe polymer was not observed.

[0026] This work should have encouraged an expert in the subject todecide not to use this type of borazine to obtain precursor polymerswith a better behaviour in spinning, since the presence of direct linksshould have been negative for spinning since the polymer was lessflexible.

[0027] On the contrary, it is observed with this invention that thistype of structure is very attractive since it is actually very close tothe structure of the ceramic. Furthermore, this arrangement limitsaggregation of cycles during polymerisation, which finally results in anon-rigid and easier to spin pseudo-linear polymer. Furthermore, it iseasy to move the amino-labile groups remaining on the polymer chainduring ceramisation.

[0028] According to the invention, thermal polymerisation of borazinewith formula (I) is carried out preferably at a final temperatureexceeding 140° C., for example from 160 to 190° C. It is possible tooperate under argon in an autogenous atmosphere, in other words toretain an atmosphere of amines that are compounds released duringthermolysis, above the polymer. Polymerisation can also be done under aninert gas flow (rare gas or nitrogen) or under a vacuum, by adaptingtemperatures and durations. Usually, since the initial borazines putinto the reactor may contain a certain quantity (up to 20% by weight) ofa synthesis solvent such as toluene, it is preferable firstly to dry themonomer under a primary vacuum before carrying out the polymerisationstep. This drying may be done at a temperature from 30 to 80° C., toeliminate the synthesis solvent.

[0029] During the polymerisation step, the eliminated volatile productscan be analysed continuously, either by pHmetry or by gaseouschromatography to control the polymerisation operation. These volatileproducts can also be trapped at low temperature and then analysed by theusual spectroscopic techniques.

[0030] Heating programs and durations and the atmospheres used depend onthe borazine used in formula (I).

[0031] After the polymerisation step, a polymer is obtained with avitreous transition temperature of less than 100° C., so that spinningis possible at temperatures less than 200° C.

[0032] The polymer can be spun using conventional techniques, usingnozzles including one hole only or several holes. The fibre leaving thenozzle may be wound onto graphite reels. Preferably, spinning is done inan inert atmosphere, for example under a nitrogen atmosphere. Thepolymer fibres are ceramised after spinning. When the reels are nottreated immediately, they can be kept in an inert chamber or under avacuum.

[0033] For ceramisation of the fibres, the temperatures, heating rates,durations and the atmosphere used are chosen as the function of theprecursor polymer used and the result to be obtained.

[0034] Preferably, ceramisation is done in two steps.

[0035] The first preceramisation step consists of heating the fibres,for example up to a temperature of less than or equal to 1000° C., andpreferably from 400 to 600° C. in an NH₃ atmosphere.

[0036] The second ceramisation step itself is carried out by increasingthe temperature of the preceramised fibres to a higher level of at least1400° C., for example from 1400° C. to 2200° C.

[0037] This step is done under a nitrogen and/or a rare gas atmospherein one or several operations, and possibly with intermediate cooling atambient temperature.

[0038] For example, this step may be carried out under a nitrogenatmosphere at a temperature from 1600 to 1800° C. and under a rare gasatmosphere beyond this temperature.

[0039] Another purpose of this invention is continuous boron nitridefibres obtained using the process described above, characterised in thatthey have an average breaking stress (σ_(R)) of 1000 to 2000 MPa and theYoung's Modulus E is between 80 and 250 GPa.

[0040] Other characteristics and advantages of the invention will bebetter seen after reading the following examples, obviously given forillustrative purposes and in no way restrictive.

DETAILED PRESENTATION OF EMBODIMENTS

[0041] The following examples illustrate the production of boron nitridefibres starting from [2,4-bis(monomethylamino)-6-dimethylamino]borazineand [2,4-bis(dimethylamino)-6-monothylamino]borazine.

EXAMPLE 1 Synthesis of[2,4-bis(monomethylamino)-6-dimethylamino]borazine

[0042] This borazine is obtained starting from trichloroborazine (TCB)by the addition of a dimethylamine equivalent for a TCB equivalent andthen, after reaction, the addition of two monomethylamine equivalents,corresponding to the following reactional diagram:

[0043] Synthesis is done in toluene. The dimethylamine is cryopumped ina TCB/toluene/Et₃N solution (0.30 M in TCB) and the reaction mix is thenadjusted to the temperature of an acetone/ice bath at −10° C. for 5hours, and stirring is then continued for another 19 hours. The sameprocedure is then continued with monomethylamine using twomonomethylamine equivalents for one TCB equivalent. The next step is tofilter the reaction mix, and the solvent is then evaporated under avacuum. The result is then a light orange viscous product containingabout 5% of toluene by mass. The product is characterised bymulti-radicals, infrared NMR and chromatography by gel permeation.

[0044] Low intensity signals are still observed in ¹H and ¹³C NMR, thatcan be assigned to the dimer with the following formula:

EXAMPLES 2 to 5 Polymerisation of(2,4-bis(monomethylamino)-6-dimethylamino]borazine

[0045] In these examples, the first step is to vacuum dry the monomer ata temperature of 50 to 80° C, and polymerisation is then carried outunder an argon atmosphere using different temperature programs.

[0046] The temperatures and durations used for polymerisations are givenin table 1. The next step is to determine the resulting polymer mass,the polymerisation rate, in other words the number of moles of nitrogenatoms released in the form of aminos per aminoborazine mole, the averagemolar mass of polymer and its vitreous transition temperature Tg.

[0047] Polymerisation conditions and the results obtained are given intable 1.

[0048] Thus, it will be noted that the vitreous transition temperaturesof polymers are not more than 90° C. and their average molar masses areof the order of 780 to 1000 g/mol.

EXAMPLES 6 to 17

[0049] In these examples, spinning, and then ceramisation of thepolymers obtained in examples 2 to 5 are carried out. For spinning, apiston with a diameter of 9.98 mm moving at a speed within the rangefrom 0.8 to 1.3 mm/min, and a nozzle with a diameter of 200 μm, areused. The spinning temperature varies from 137 to 192° C. At the exitfrom the nozzle, the fibres are wound onto a graphite reel with adiameter of 50 mm in examples 6 to 14, and onto a graphite reel with adiameter of 100 mm in examples 15, 16 and 17. The spooling speed canvary from 1.5 revolutions/second to 25 revolutions/second.

[0050] Spinning conditions and the initial polymers are given in tables2 to 4. After spinning, the polymer fibres are ceramised under theconditions described below.

[0051] Ceramisation A:

[0052] a) Preceramisation: heat up to 600° C. at a rate of 25° C./h,under NH₃.

[0053] b) Ceramisation:

[0054] Heat from 600 to 1100° C., at a rate of 100° C./h under N₂.

[0055] Hold at 1100° C., under N₂ for 90 minutes.

[0056] Cool to ambient temperature.

[0057] Heat up to 1400° C., at a rate of 600° C./h, under N₂.

[0058] Hold at 1400° C, under N₂, for 1 hour.

[0059] Heat from 1400° C. to 1600° C., at a rate of 600° C./h, under N₂.

[0060] Hold at 1600° C., under N₂ for 1 hour.

[0061] Heat from 1600 to 1800° C., at a rate of 600° C./h under N₂.

[0062] Hold at 1800° C., under N₂ for 1 hour.

[0063] Ceramisation B:

[0064] a) Preceramisation: heat up to 600° C. at a rate of 25° C./h,under NH₃.

[0065] b) Ceramisation:

[0066] Heat from 600 to 1100° C., at a rate of 100° C./h under N₂.

[0067] Hold at 1100° C., under N₂ for 90 minutes.

[0068] Cool to ambient temperature.

[0069] Heat up to 1400° C., at a rate of 600° C./h, under N₂.

[0070] Hold at 1400° C., under N₂, for 1 hour.

[0071] Heat from 1400° C. to 1600° C., at a rate of 600° C./h, under N₂.

[0072] Hold at 1600° C., under N₂ for 1 hour.

[0073] Ceramisation C:

[0074] a) Preceramisation

[0075] Heat up to 375° C. at a rate of 10° C./h, under NH₃.

[0076] Heat from 375° C. to 600° C. at a rate of 15° C./h, under NH₃.

[0077] b) Ceramisation:

[0078] Heat from 600 to 1100° C., at a rate of 100° C./h under N₂.

[0079] Hold at 1100° C., under N₂ for 90 minutes.

[0080] Cool to ambient temperature.

[0081] Heat up to 1400° C., at a rate of 600° C./h, under N₂.

[0082] Hold at 1400° C., under N₂, for 1 hour.

[0083] Heat from 1400° C. to 1600° C., at a rate of 600° C./h, under N₂.

[0084] Hold at 1600° C., under N₂ for 1 hour.

[0085] Ceramisation D:

[0086] a) Preceramisation: heat up to 600° C. at a rate of 25° C./h,under NH₃.

[0087] b) Ceramisation:

[0088] Heat from 600 to 1100° C., at a rate of 100° C./h under N₂.

[0089] Hold at 1100° C., under N₂ for 90 minutes.

[0090] Cool to ambient temperature.

[0091] Heat up to 1400° C., at a rate of 600° C./h, under N₂.

[0092] Hold at 1400° C., under N₂, for 1 hour.

[0093] Heat from 1400° C. to 1600° C., at a rate of 600° C./h, under N₂.

[0094] Hold at 1600° C., under N₂ for 1 hour.

[0095] Heat from 1600 to 1800° C., at a rate of 600° C./h under N₂.

[0096] Hold at 1800° C., under N₂ for 1 hour.

[0097] Heat from 1800 to 2000° C., at a rate of 600° C./h under argon.

[0098] Hold at 2000° C., under argon, for 1 hour.

[0099] After obtaining ceramised fibres, the diameter of the fibres,their ultimate stress σ_(R) (in MPa) and their Young's modulus E (inGPa) are determined as follows.

[0100] The ultimate stress σ_(R) is determined on about fifty singlefilaments with a test piece length of 1 cm. The ultimate tests areanalysed using Weibull's model in which the ultimate stresses aredetermined for a failure of probability equal to 0.5. An average valueof the distribution of the elongations to rupture (ε_(R)) is defined,and this value is used to calculate the median value of the distributionof ultimate stresses (σ_(R)) at a survival probability of 0.5. TheYoung's Modulus or the Modulus of Elasticity E can then be determined.

[0101] Spinning and ceramisation conditions and the results obtained aregiven in tables 2 to 4.

[0102] Note that the values of the modulus E of the boron nitride fibresobtained are very high and vary from 150 to 244 GPa, and the ultimatestresses σ_(R) are also very high.

[0103] Thus, the use of the polymer obtained from[2,4-bis(monomethylamino)-6-dimethylamino]-borazine) according to theinvention can give very attractive results and produce boron nitridefibres with high performances.

EXAMPLE 18 Preparation of boron nitride Fibres from[2,4-bis(dimethylamino)-6-monomethylamino] borazine

[0104] a) Synthesis of the Monomer

[0105] The monomer is obtained in the same way as the monomer in example1, but by adding two dimethylamine equivalents for one equivalent ofTCB, and then after the reaction, a single equivalent ofmonomethylamine. The monomer is characterised by multi-radicals,infrared and chromatography NMR by gel permeation.

[0106] b) Polymerisation

[0107] Thermal polymerisation of the monomer is done under the followingconditions:

[0108] 50° C.-1 h00 (under argon),

[0109] 80° C.-1 h00 (under argon),

[0110] 130° C.-1 h30 (under argon),

[0111] 160° C.-13 h00 (under argon),

[0112] 175° C.-4 h00 (under argon),

[0113] 180° C.-4 h00 (under argon), and

[0114] 185° C.-2 h00 (under argon).

[0115] The resulting progress is 22%. The vitreous transitiontemperature of the polymer is of the order of 50° C.

[0116] The average molecular mass by weight is 500 g/mol.

[0117] c) spinning and ceramisation

[0118] The polymer is spun as in examples 1 to 17, under the followingconditions:

[0119] T_(spinning) . . . : 119° C.

[0120] Piston speed . . . : 0.8 to 1 mm/min

[0121] Spooling speed . . . : 1.5 rps

[0122] The diameter of the raw fibres is 21 μm.

[0123] The next step is ceramisation of the fibres using ceramisation A.The result is 14.8 μm diameter ceramised fibres with the followingmechanical characteristics.

[0124] σ_(r): 512 MPa

[0125] E: 57 GPa

REFERENCES

[0126] [1]: R. T. PAINE et al, Chem. Rev., 90, 1990, pp. 73-91.

[0127] [2]: C. K. Narula et al in Chem Mater, 2, 1990, pp. 384-389.

[0128] [3]: EP-A-0 342 673.

[0129] [4]: FR-A-2 695 645.

[0130] [5]: T. Wideman et al, Chem. Mater., 10, 1998, pp. 412-421.

[0131] [6]: B. Toury et al in Main Group Met. Chem. 22, 1999, pp.231-234. TABLE 1 EX 2 3 4 5 Polymerisation 130° C. - 1h00 80° C. - 30min 80° C. - 1h (arg) 80° C. - 30 min (arg) (arg) (arg) 130° C. - 1h00140° C. - 1h00 (arg) 130° C. - 1h20 (arg) (arg) 160° C. - 160° C. - 170°C. - 14h30 (arg) 160° C. - 16h00 (arg) 17h00 (arg) 16h00 (arg) 170° C. -2h30 (arg) 170° C. - 1h30 170° C. - 2h20 180° C. - 40 min 175° C. - 1h30(arg) (arg) (arg) (arg) Monomer mass m_(m) = 7.6 g m_(m) = 11.0 g m_(m)= 11.5 g m_(m) = 10.7 g Polymer mass m_(p) = 6.4 g m_(p) = 9.1 g m_(p) =9.3 g m_(p) = 8.6 g Polymerisation 0.72 0.78 1.17 0.90 rate Averagemolar MW = 780 g/mol MW = 840 g/mol MW = 1000 g/mol MW = 1000 g/mol massT_(g) T_(g) = 56° C. T_(g) = 60° C. T_(g) = 90° C. T_(g) = 65° C.

[0132] TABLE 2 EX 6 7 8 9 Polymer Example 2 Example 3 Example 3 Example4 Spool diameter 50 mm 50 mm 50 mm 50 mm T_(spinning) 137° C. 152° C.153° C. 192° C. S_(spooling) 1.5 rev/sec 8 rev/sec 12 rev/sec 5.2rev/sec S_(piston) 1.2 mm/min 0.9- 0.8 mm/min 0.9- 1 mm/min 1 mm/minCeramisation A A A A φ ceramised 10.7 11.6 11.4 24.1 fibres (μm) σ (MPa)685 851 1241 423 MPa E (GPa) 170 149 218 77 GPa

[0133] TABLE 3 EX 10 11 12 13 14 Polymer Example 5 Example 5 Example 5Example 5 Example 5 Spool 50 mm 50 mm 50 mm 50 mm 50 mm diameterT_(spinning) 163° C. 164° C. 164° C. 164° C. 164° C. S_(spooling) 25rev/sec 17 rev/sec 25 rev/sec 17 rev/sec 25 rev/sec S_(piston) 1-1.3 1mm/min 1 mm/min 0.9 mm/min 0.9 mm/min mm/min Ceramisation A B B C C φceramised 11.2 11.2 10.7 11.5 9.9 fibres (μm) σ (MPa) 1177 1287 1367 9001157 E (GPa) 193 175 209 192 214

[0134] EX 15 16 17 Polymer Example 5 Example 5 Example 5 Spool diameter100 mm 100 mm 100 mm T_(spinning) 164° C. 164° C. 164° C. S_(piston) 0.9mm/min 0.9 mm/min 0.9 mm/min S_(spooling) 20 rev/sec 10 rev/sec 7rev/sec Ceramisation D D A φ ceramised 6.4 6.7 8.0 fibres (μm) σ (MPa)1189 1242 819 E (GPa) 166 244 186

1. Process for manufacturing boron nitride fibres by spinning of aprecursor polymer and ceramisation of the polymer fibres obtained byspinning, characterised in that the precursor polymer is obtained bythermal polymerisation of a borazine of formula (I):

in which R¹, R³, R⁴ and R⁵ that may be identical or different, representan alkyl, cycloalkyl or aryl group, and R² represents a hydrogen atom oran alkyl, cycloalkyl or aryl group:
 2. Process according to claim 1, inwhich R² represents a hydrogen atom.
 3. Process according to claim 2, inwhich borazine complies with formula (I) in which R¹, R³, R⁴ and R⁵represent the methyl group.
 4. Process according to claim 1, in whichborazine complies with formula (I) in which R¹, R², R³, R⁴ and R⁵represent the methyl group.
 5. Process according to any one of claims 1to 4, in which thermal polymerisation is done at a final temperature of160 to 190° C. under an inert atmosphere.
 6. Process according to claim1, in which the precursor polymer is spun under an inert atmosphere at atemperature of less than 200° C.
 7. Process according to any one ofclaims 1 to 6, in which the polymer fibres are transformed into boronnitride fibres by carrying out the following steps in sequence. a)heating in an NH₃ atmosphere up to a temperature of less than or equalto 110° C., and b) heat treatment in a nitrogen and/or rare gasatmosphere at a temperature of at least 1400° C.
 8. Process according toclaim 7, in which the heat treatment in step b) is carried out under anitrogen atmosphere at a temperature of 1600 to 1800° C. and under arare gas atmosphere beyond this temperature.
 9. Continuous boron nitridefibres obtained by the process according to any one of claims 1 to 8,characterised in that the median ultimate stress σ_(R) is equal to 1000to 2000 MPa and the Young's modulus E varies from 80 to 250 GPa.